The present invention relates to an apparatus for damping and monitoring emissions from a laser diode, particularly, from a vertical cavity surface emitting laser (VCSEL). Also, the present invention relates to an opto-electrical assembly (optical engine) including the apparatus for damping and monitoring emissions from a laser diode.
In order to support the communication requirements of high-speed data transmission applications, optical links are used when links via an electrical wire have a too low bandwidth. When using an optical link for transmitting a signal from a first electronic component to a second electronic component, the electric signal to be transmitted is first converted into an optical signal, then the optical signal is coupled into an optical fiber via an optical transmitter and transmitted to the second electronic component via the optical fiber. On the other hand, when the first electronic component uses an optical link for receiving an optical signal from the second electronic component, the first electronic component converts the received optical signal into an electrical signal before further processing.
FIG. 1 shows an opto-electrical assembly (also denoted as optical engine) for converting an electric signal into an optical signal and vice-versa. The optical engine 100 includes an optical carrier 101, an array of four laser diodes 103 (for instance, vertical cavity surface emitting lasers, abbreviated VCSEL), a driver IC 102 including an array of four drivers for driving the four laser diodes 103, an array of four photodiodes 104, and an integrated circuit (IC) including an array of four transimpedance amplifiers 105 (abbreviated TIAs) for amplifying the output signals of the four photodiodes 104. The driver IC 102, the array of laser diodes 103, the array of photodiodes 104 and the IC including the TIAs 105 are all mounted on the optical carrier 101. The optical carrier 101 essentially includes a glass substrate which is transparent for the light/laser emission of the optical signals.
Each driver of the driver array 102 receives at its input terminals 106 an electric signal, converts the received electrical signal into an electric driver signal for driving a respective laser diode of the array of laser diodes 103. Each laser diode of the array of laser diodes 103 converts the driving signal received at its inputs to an optical signal, which is output to, for instance, an optical fiber (not shown in FIG. 1). Each photodiode of the array of photodiodes 104 receives an optical signal from, for instance an optical fiber, converts the received optical signal into an electrical signal, and outputs this to the inputs of a respective TIA of the array of TIAs 105.
When using laser diodes for converting electrical signals into optical signals, the optical engine 100 has to meet standard eye safety regulations to avoid eye damage to an operator/user. Therefore, the output power of each optical signal which leaves the optical engine 100 must not exceed eye safety limits. The power level of laser emissions output by vertical cavity surface emitting lasers (VCSELs) normally exceeds the eye safety limits. Therefore, damping/limiting/attenuating of the laser emissions output by VCSELs is required. In order to damp the optical power of the laser emission leaving the optical engine 100, for instance, towards an optical fiber, a damping layer is used in known optical engines. This damping layer is interposed in the optical path of the laser emissions of the VCSELs. Typically, the damping layer is deposited on a surface of the glass substrate and integrated in the optical carrier 101. The damping layer also reduces optical reflections toward VCSEL
Furthermore, in fiber optic communication systems a nearly constant output power of the VCSELs is desired. However, during operation, ambient temperature changes and aging of the device can result in fluctuations in the output power of the VCSELs. Therefore, monitoring of the output power of the VCSELs is advantageous.
To this end, document U.S. Pat. No. 6,037,644 discloses an apparatus for monitoring emissions from VCSELs. FIG. 2 shows a cross sectional view of this apparatus. It comprises: a glass substrate 137, an amorphous silicon layer 136 on the glass substrate 137, a separate substrate 131 having selected areas being provided with the VCSELs 132, and a flex circuit layer 134 interposed between the amorphous silicon layer 136 and the separate substrate 131. The amorphous silicon layer 136 has selectively doped areas that provide PN junctions 139 of photovoltaic devices and a sufficiently small optical absorption coefficient, within an optical wavelength range, that limits absorption of a corresponding small fractional amount of light being emitted by the VCSELs 132. The amorphous silicon layer 136 and the glass substrate 137 are sufficiently thin to be transmissive of such light therethrough, excluding the small fractional amount of such light being absorbed by the photovoltaic devices. The photovoltaic devices are on top of the VCSELs. The VCSELs 132 are further constructed and arranged to emit such light having a wavelength range that corresponds to the wavelength range at which the amorphous silicon layer 136 has the sufficiently small absorption coefficient, whereby, a substantial portion of such light is transmitted through the photovoltaic devices, and a small fraction of such light is absorbed by the doped areas 139 of the amorphous silicon layer 136 to produce photovoltaic currents from the photovoltaic devices as a measurement of output power of the VCSELs 132. The flex circuit layer 134 is transparent and Includes on its upper and lower surface areas of semitransparent contact metallization 135. The amorphous silicon layer 136 has semitransparent contact metallization areas 140 being in contact with areas of semitransparent contact metallization 135 of the upper surface of the transparent flex circuit layer 134; and the separate substrate 131 has respective semitransparent contact metallization areas 132 being in contact with the areas of semitransparent contact metallization 135 of the lower surface of the flex circuit layer 140.
The apparatus for monitoring laser emissions from VCSELs shown in FIG. 2 requires PN junctions 139 (i.e. photodiodes) for monitoring the output power of the VCSELs 132. However, implementing these PN junctions complicates the manufacturing process of the apparatus and consequently enhances its manufacturing costs. Also, the structure shown in FIG. 2 requires the flex circuit layer 134 for providing electrical contacts to the VCSELs 132 and the photodiodes 139, which further enhances the manufacturing costs.